1. Field of the Invention
The present invention relates to an electrophotographic color image forming apparatus using a plurality of developing devices.
2. Description of the Related Art
As full-color image forming apparatuses, image forming apparatuses of a rotary type in which a plurality of developing devices are supported by a rotating support body (rotary) are known. In an image forming apparatus of this rotary type, one of the developing devices is placed at a position opposing a photosensitive member (developing position) by rotating the rotary in a predetermined direction, and develops a latent image on the photosensitive member into a visible toner image. The toner image is then transferred onto an intermediate transfer medium. By repeating these operations while rotating the rotary and sequentially switching among the developing devices, toner images of a plurality of colors are superimposed to form a color image. When formation of the color image is completed, the rotary returns to a preset standby position and stands by until the next image signal is given.
It is known that the above-described full-color image forming apparatus is operable in a multicolor mode (color mode) in which an image is formed in full color (yellow, magenta, cyan, and black) and a monochromatic mode (mono mode) in which an image is formed in a single color (black). In a color mode, development is sequentially performed in yellow, magenta, cyan, and black colors in this order while rotating the rotary in a predetermined direction. By thus repeating image forming operations, such as development and transfer, a plurality of times, a color image is formed. When image formation is performed in a mono mode, only the black developing device is moved to the developing position, while the developing devices for the other colors are not moved to the developing position and do not perform development. This makes the image forming speed higher than in the color mode. Moreover, it is possible to suppress wear of the development devices that are not used.
Japanese Patent Laid-Open No. 2005-003865 discloses an apparatus that is used as a monochromatic image forming apparatus when a developing-agent container is loaded in one of a plurality of loading/unloading portions, and as a color image forming apparatus when a plurality of developing-agent containers are loaded in the loading/unloading portions. The home position of a rotating body adopted when the image forming apparatus is used as the monochromatic image forming apparatus is different from that adopted when the image forming apparatus is used as the color image forming apparatus.
With recent diversification of users, image formation has been performed in various image patterns and with various types and sizes of paper. To respond to such image formation, it is known that image formation is interrupted by a special operation at a predetermined timing even during continuous printing. When such a special operation is performed, a developing operation is, of course, stopped temporarily. In this case, the rotary waits for start of the next image formation at a developing standby position where the developing device is away from the developing position.
When image formation is interrupted and the rotary is set at the developing standby position in the above-described image forming apparatus, much time may be taken to rotate the developing device to the developing position in the next image forming operation, depending on the developing standby position. For example, in a case in which the yellow developing device stands by at a position near the developing position, when the next image forming operation is to be performed in a mono mode, much time is taken to move the black developing device to the developing position. In the apparatus disclosed in Japanese Patent Laid-Open NO. 2005-003865, the home position is changed in accordance with the number of developing-agent containers loaded in the loading/unloading portions. Therefore, when the developing operation is interrupted during image formation in a state in which the developing-agent containers are loaded in a plurality of loading/unloading portions, it is difficult to change the home position.